Solid desiccant cooling systems of the aforementioned kind have been proposed in a variety of configurations. In the basic arrangement shown in FIG. 1, fresh (outside) air 1 supplied by a supply air fan is dehumidified in a rotary desiccant wheel 3. In this near adiabatic drying process, the air is unavoidably warmed. A heat recovery heat exchanger 4 is used to cool the warm dry air back down to near ambient temperature. The resulting pre-cooled, dry air stream is then further cooled to temperatures below ambient using an evaporative cooling process 6 before it is introduced into the occupied space (supply air 7) to provide the desired space conditioning.
Regeneration of the desiccant wheel is required to ensure a continuous drying process. Regeneration is achieved by passing hot air through one side of the desiccant wheel. Moisture removed from the desiccant wheel is exhausted with, the regeneration air stream exiting the desiccant wheel.
Regeneration air can be sourced from the occupied space (return air 8) or from outside ambient (fresh air). Regeneration air is first evaporatively cooled in coolers 9 before it is pre-heated in the heat recovery heat exchanger 4. This minimises the supply air temperature before the supply air evaporative cooling process and maximises the regeneration air temperature before it is further heated in a heating coil 10 with externally supplied heat. The heated return air is then passed through the regenerator side of the desiccant wheel 3 before being exhausted by regeneration air fan 11 as exhaust air 12.
Desiccant cooling is primarily found in commercial and larger-scale installations, especially where higher humidity is a significant issue, for example in supermarkets and ice-skating venues. The technology is not found in residential applications to any significant extent, notwithstanding a number of potential advantages: robustness, easy maintenance and efficient operation with low temperature heat such as that from roof-mounted solar collectors. Solar desiccant cooling systems have been evaluated in a number of publications (including S. D. White et al. “Indoor temperature variations resulting from solid desiccant cooling in a building without thermal back-up”, International Journal of Refrigeration 32 (2009), 695-704; and Rowe et al. “Preliminary findings on the performance of a new residential solar desiccant air-conditioner”, Proc. Eurosun 2010, Graz, October 2010).
The limited application of desiccant cooling systems has arisen from disadvantages of the basic arrangement described above. This process suffers from (i) high parasitic fan power consumption due to the large pressure drops across the desiccant wheel and heat recovery wheel, (ii) bulkiness (due to the presence of two fans to respectively drive air on the supply and regeneration sides), (iii) cost and (iv) unsuitability for autonomous cooling with an intermittent heat source (due to the inability to achieve significant cooling when heat is not available for regenerating the desiccant wheel).
Previously, it has been proposed to address these disadvantages, at least to an extent, by replacing the heat recovery heat exchanger, employed to cool the warm dry air on the supply side back down to near ambient temperature and to pre-heat the regeneration air, with an indirect evaporative cooler on the supply side.
It has been realised that the earlier mentioned previous proposal to replace the heat recovery heat exchanger with an indirect evaporative cooler on the supply side presented an opportunity to substantially eliminate the pressure imbalances between the supply and regeneration sides of the desiccant cooling circuit. Specifically, the supply air and regeneration air streams both flow through                a single heat exchanger process and        a single desiccant wheel pass of equal effective length,        
For the purposes of the present invention, the effective length along a pathway is the length of the ducting plus the equivalent length of a ducting, which produces the same pressure drop as a unit operation(s) (e.g. heat exchanger and/or desiccant wheel) along the pathway. For example, a pathway comprising ducting of length L and a desiccant wheel (Ld) has an effective length of L+Ld, where Ld is the length of ducting which produces the same pressure drop as the desiccant wheel. Therefore, by definition pathways of similar effective length have pressure drops of a similar magnitude.
As a result, the flow velocities and pressure drops on the supply and regeneration air sides are each of a similar magnitude. This inherent balance of pressure drop between the supply air and regeneration air enables a single source of air to divide and flow equally (or substantially equal) between the supply and regeneration air ducts without need for pressure balancing/reducing dampers or other control devices.
Consequently, the conventional pair of fans used in the conventional desiccant cooling process can be replaced with a single fan supplying air to both the supply and the regeneration sides, without resorting to pressure reducing dampers for controlling flow between the supply and regeneration air sides. It has been further appreciated that one fan instead of two would reduce the bulk and cost of the system, and eliminating pressure reducing dampers would reduce parasitic fan power.
Shapiro (US 2009/0145140 A1, 2009) proposes a cycle with a single fan. However, this cycle is not inherently balanced. In the Shapiro cycle, the air to be dried passes through the desiccant wheel and a heat exchanger, while the regeneration air stream passes through the desiccant wheel and four heat exchangers. Consequently the Shapiro cycle needs pressure reducing dampers to balance the flows and pressures between the supply and regeneration sides, leading to unacceptably high fan energy consumption compared with the conventional two fan process.
Furthermore the Shapiro cycle does not provide means for transitioning between alternative modes of operation when utilizing a single fan.
It has also been recognised that environmental conditions and location have an effect on the qualities of the air to be conditioned and the reconditioned air. These qualities include humidity and temperature. Thus a solid desiccant cooling system which is designed for efficient use under one set of climactic or environmental conditions may not be efficient at all times of the year and may not be capable of use in a wide range of locations and circumstances. Thus the ability to transition to different modes of operation depending for seasonal or other circumstances may be advantageous for efficient operation.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.
It is an object of the invention to provide one or more modifications of solid desiccant cooling processes of the kind earlier described that at least in part overcome the afore described disadvantages.